The invention relates to a protective element for an electrochemical accumulator device comprising at least one storage cell having first and second electrodes separated by an electrolyte, and to a process for making such a protective element.
German patent document 44 09 268.7-45 discloses an electrochemical accumulator having a plurality of serially interconnected individual cells, each of which has two spatially separated electrodes, with at least one electrolyte and protective element being disposed therebetween. The protective element made of varistor material serves, on the one hand, to maintain the gap between the electrodes and, on the other hand, to provide against an overcharge of the cell.
The varistor material has a nonlinear resistance, acting as an insulator below a specific voltage and becoming conductive above said voltage. That is, similar to a Zener diode, the protective element possesses a breakdown voltage at which the change from insulator to conductor characteristics occurs. Advantageously, the critical voltage in this case is lower than or equal to the overcharging voltage or destructive voltage beyond which an element of an individual cell, generally the electrolyte, suffers irreparable damage. The use of a varistor material as a protective element is critical, however, owing to the difficulty of setting the breakdown voltage, since the latter generally is a function of the layer thickness of the varistor material. The layer thickness of the protective element in turn, however, enters directly into the size of the individual cell, and as a result the overall volume of designs protected with varistor materials is considerable.
The scientific article "Synthesis of Submicrometer Crystals of Aluminum Oxide by Aqueous Intravesicular Precipitation" in Journal of Colloid and Interface Science, Vol. 135, No. 2, Mar. 15th, 1990, pages 531-538, discloses a method for preparing aluminum oxide grains which have a mean diameter of approximately 10-20 nm. To prepare such grains, an aluminum salt solution is enclosed in phospholipid vesicles, the Al ions in the outside medium are removed, and by means of a marked increase in the pH, precipitation of aluminum oxide is triggered in the voids formed by the vesicles. After the formation of the desired substance, the vesicles are removed and grains in the desired order of magnitude have been obtained. In addition, other biochemical hollow bodies such as, e.g. micelles and ferritin, have been described as matrices for preparing such vesicles.
U.S. Pat. No. 3,821,686 and European patent document 50 091 disclose protective elements made of granular varistor material, which are disposed directly between two conductors and may therefore, in particular cases, have to be designed to be very small.
German patent document 37 21 754 A1 discloses a protective element for an electrochemical accumulator comprising at least one individual cell, each individual cell having two spatially separated electrodes between which an electrolyte and a protective element consisting of varistor material is situated. The protective element comprises varistors which regularly consist of granular varistor material and which--owing to the varistor characteristics--prevent overcharging.
German patent document 42 29 437 C1 discloses an electrochemical accumulator whose electrodes are separated from one another by means of spacers.
German patent document 36 19 620 A1 discloses varistor bodies consisting of fine-grained varistor material. It further discloses the interrelationship that, owing to the more numerous grain boundaries per unit length, varistors having a smaller thickness or a smaller volume can be used.
German patent documents 43 20 836 A1, 38 26 356 A1 and 25 26 137 C2 disclose the preparation of varistor materials. These publications disclose different starting materials and different intermediates and the further processing thereof.
German patent document 43 20 836 A1 discloses, in particular, the preparation of finely particulate mixed oxide powders, whose grain size is usually below 1 .mu.m.
German patent document 38 26 356 A1 discloses, in particular, the uniformity of the particle size within narrow limits, the particle size overall being low, below 1 .mu.m.
German Patent 25 26 137 C2 discloses the precipitation of intermediates of varistor material.
German patent document DE-B-10 46 153 discloses granular varistor materials for which, in regions outside contiguous grain boundaries, a binder is provided which presses the grains against one another.
German patent document 34 09 815 A1 discloses ceramic materials in porous form.
The object of the present invention is to provide protective element which permits individual cells protected therewith to have as low an overall volume as possible, while enabling the fabrication of the protective element with the smallest possible manufacturing tolerances in terms of overall dimensions and of breakdown voltage.
Furthermore, it is another an object of this invention to provide a process for fabricating the protective elements according to the invention.
The object is achieved by the protective element according to the invention, which comprises granules of varistor material having a mean diameter of less than 1.0 .mu.m, more than 60 percent of the granules having a size which varies form the average by less than 40 percent. The granular varistor material which forms the protective element is fabricated with the aid of biochemical hollow bodies, in particular of vesicles. In this manner, the grains of the varistor material can be produced below the .mu.m region and within narrow interval limits. The grain size of the grains of the varistor material are prepared by means of an intermediate which forms the varistor material by calcination, within narrow interval limits and in a form which at least roughly corresponds to a spherical shape. Due to the resulting small grain size, it is possible, on the one hand, for the touching surface of the individual grains of the varistor material (and hence the breakdown voltage of the protective element) to be adjusted very accurately, while at the same time keeping the layer thickness of the protective elements small. Owing to the process, it is furthermore advantageously possible, by means of the biochemical hollow bodies for the grain size of the intermediate and thus of the varistor material to be varied, virtually at will, from the nm range up to the .mu.m range. This advantageously enables the use in individual cells of different configurations in terms of layer thickness.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.